Citric acid can be produced in commercial quantities by the fermentation of carbohydrate materials using various strains of citric acid-producing fungi. Certain strains of Aspergillus and Penicillium have proved to be useful, for example Asperigullus niger; A. clavatus; A. wentiri; A. luchuensis; Penicillium citrinum; and P. luteum.
One form of fermentation process employed in the art for citric acid production involves fermentation by A. niger in submerged culture. The fermentation substrate can be a carbohydrate material such as invert sugars, partially-refined sucrose sources, glucose and starch hydrolyzates obtained from highly refined starches. For example, corn starch can be hydrolyzed into sugars such as glucose by treatment with acids such as hydrochloric acid. Corn starch can also be enzymatically converted by alpha-amylase and amyloglucosidase into such sugars. After the starch is converted into sugar, the resulting sugar-containing material is subjected to the action of a citric-acid producing strain of a fungus in the presence of nutrients, under conditions conducive to fermentation, to produce citric acid.
During the hydrolysis of starch into sugars, byproducts are formed, including reducing (reversion) sugars such as amino sugars, e.g., glucosamine; oligosaccharides, e.g., isomaltose and glucose. The major reducing sugar present apparently is glucose. These reducing sugars are carried over and are present in subsequent steps, including the production of citric acid, subsequent purification of citric acid and conversion of citric acid into trisodium citrate.
One major market for trisodium citrate is its use in detergents as a "builder". Some dyes present in commercially available detergents are susceptible to color fading in the presence of reducing sugars.